According to FIG. 1 a thermal power plant 10 comprises a heavy-duty gas turbine 12, a generator 13 driven directly by the gas turbine by means of a shaft 14, and being connected to an electrical grid 17 via an electronic decoupling apparatus 15 and a conventional step-up transformer 16.
With the electronic decoupling apparatus 15, the frequency of the generator 13 may be different from the grid frequency FG of the electric grid 17 in order to improve the operational characteristics and service life of the gas turbine 12 and generator 13.
This decoupling from the grid frequency will result in this particular type of power plant 10 in being not able to inherently provide inertial power in response to a grid frequency disturbance event as in conventional power plant comprising synchronous generators. The impact of lack of inertial response may contribute to increase instability in the electrical grid and may not meet the grid requirements as specified in the transmission code.
Document US 2010/0031667 A1 discloses a method for operating a power station, with turbine shafting, that includes a gas turbine and a generator driven directly by the gas turbine and that generates alternating current with an operating frequency. The output of the generator is connected with an electrical grid with given grid frequency. An electronic decoupling apparatus or variable electronic gearbox is arranged between the generator and the grid, whereby the decoupling apparatus decouples the operating frequency from the grid frequency. Increased service life of the station and reduced emissions are achieved in that, when there are longer-lasting changes in the grid frequency, the mechanical or aerodynamic rotational speed of the gas turbine is kept constant and the output of the gas turbine is adjusted without a delay.
Document US 2010/0032964 A1 discloses a method for operating a power station with turbine shafting including a gas turbine and a generator that is driven directly by the gas turbine and that generates alternating current with an operating frequency. The output of the generator is connected to an electrical grid with a given grid frequency. An electronic decoupling apparatus or variable electronic gearbox is arranged between the generator and the grid. The decoupling apparatus decouples the operating frequency from the grid frequency. In the event of a temporary over-frequency or under-frequency event in the electrical grid, the mechanical rotational speed of the gas turbine is decreased more than the grid frequency during an under-frequency event of the electrical grid and is increased more than the grid frequency during an over-frequency event of the electrical grid.
In both cases, inertia does not play any role.